Recently, with the advent of a highly information-based society, materials satisfying both heat resistance and transparency have become desired in the field of optical communications including optical fibers, optical waveguides, etc., and in the field of display devices including liquid-crystal orientation films, color filters, etc.
In the field of display devices, an alternative technology of employing plastic substrates that are lightweight and are excellent in flexibility, in place of glass substrates, and a development of displays capable of being bent or rolled up are now under way. However, for example, when an electronic element composed of an inorganic material is formed on a film, the film having the inorganic element formed thereon may bend and, as the case may be, the inorganic element may often peel away from the film, since the inorganic material and the film significantly differ in point of the linear expansion coefficient. Accordingly, it is desired to develop a resin material for films having both transparency and heat resistance and having a low coefficient of linear thermal expansion.
Polyimide has excellent heat resistance and additionally has other excellent properties of mechanical characteristics, chemical resistance, electric characteristics and the like, and therefore films formed of a material of polyimide are widely used in various fields of molding materials, composite materials, electric and electronic components, display devices, etc. However, those films are further required to have higher transparency and dimensional stability than ever.
In general, it is known that polyimides having the polymer chains which are more rigid and have a higher linearity have a lower coefficient of linear thermal expansion, and for improving the dimensional stability of polyimides by lowering the coefficient of linear thermal expansion thereof, various structures of both acid dianhydrides and diamines that are raw materials of polyimides have heretofore been proposed.
PTL 1 discloses a polyimide precursor to be formed through reaction of a diamine component containing a fluorine-containing aromatic diamine and a specific aliphatic diamine, and an acid dianhydride component containing an aliphatic tetracarboxylic dianhydride and an aromatic tetracarboxylic dianhydride. However, the polyimide film formed from the polyimide precursor has a light transmittance at a wavelength of 400 nm of 80% or so when the thickness thereof is 10 μm, that is, the film could not be said to have good transparency.
PTL 2 discloses a polyimide film formed by casting a solution prepared by mixing a solution of a specific polyamic acid having an aromatic ring with a solution containing a dehydrating catalyst and an imidating agent having a boiling point of not higher than 180° C., onto a support, and says that the film is excellent in heat resistance and has a low linear expansion coefficient. However, the polyimide film has a YI index of 15 or so when having a thickness of 50 μm, that is, the yellowness of the film is strong, and the transparency thereof is expected to be insufficient.
PTL 3 discloses a copolyimide having a biphenyl structure and having excellent transparency, high heat resistance, a high glass transition temperature, a low coefficient of linear thermal expansion and good bending resistance. However, the film formed of the copolyimide has a light transmittance at a wavelength of 400 nm of at most 80% or so, when having a thickness of 10 μm, that is, the film could not be said to have good transparency.